The invention relates generally to a memory device, and more particularly, to a memory device that emulates dynamic random access memory (DRAM).
Magnetic random access memory (MRAM) is a new class of non-volatile memory. Unlike volatile memory, such as dynamic random access memory (DRAM) that loses the stored information when power is interrupted, non-volatile memory can retain the stored information even when powered off.
An MRAM device normally comprises an array of memory cells, each of which includes at least a magnetic memory element and a selection element or transistor coupled in series between appropriate electrodes. Upon application of an appropriate current or voltage to the magnetic memory element in the programming step, the electrical resistance of the magnetic memory element would change accordingly, thereby switching the stored logic in the respective memory cell.
The magnetic memory element typically includes at least a magnetic reference layer and a magnetic free layer with a non-magnetic tunnel junction layer interposed therebetween, thereby forming a magnetic tunneling junction (MTJ). The magnetic reference layer has a fixed magnetization direction and may be anti-ferromagnetically exchange coupled to a magnetic pinned layer, which has a fixed but opposite or anti-parallel magnetization direction. Upon the application of an appropriate current through the MTJ, the magnetization direction of the magnetic free layer can be switched between two directions: parallel and antiparallel with respect to the magnetization direction of the magnetic reference layer. The non-magnetic tunnel junction layer is normally made of an insulating material with a thickness ranging from a few to a few tens of angstroms. When the magnetization directions of the magnetic free and reference layers are substantially parallel, electrons polarized by the magnetic reference layer can tunnel through the insulating tunnel junction layer, thereby decreasing the electrical resistance of the MTJ. Conversely, the electrical resistance of the MTJ is high when the magnetization directions of the magnetic reference and free layers are substantially anti-parallel. Accordingly, the stored logic in the magnetic memory element can be switched by changing the magnetization direction of the magnetic free layer.
Based on the relative orientation between the magnetic reference and free layers and the magnetization directions thereof, an MTJ can be classified as in-plane MTJ or perpendicular MTJ. The magnetic reference and free layers of an in-plane MTJ have magnetization directions that lie in planes substantially parallel to layer planes thereof, whereas the magnetic reference and free layers of a perpendicular MTJ have magnetization directions that are substantially perpendicular to layer planes thereof.
The stability of the magnetic free layer in the MTJ enables an MRAM device to have years of data retention but may prevent the same device to program at a high speed comparable to that of DRAM, thereby limiting MRAM's usage in memory applications.
For the foregoing reasons, there is a need for a memory device that includes MRAM and emulates DRAM by operating at a high input/output (I/O) speed while retaining the non-volatility characteristic of the MRAM.